1. Field
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive that characterizes misaligned servo wedges.
2. Description of the Prior Art and Related Information
A huge market exists for disk drives for mass-market computing devices such as desktop computers and laptop computers, as well as small form factor (SFF) disk drives for use in mobile computing devices (e.g. personal digital assistants (PDAs), cell-phones, digital cameras, etc.). To be competitive, a disk drive should be relatively inexpensive and provide substantial capacity, rapid access to data, and reliable performance.
Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable transducer head for reading and writing data. The HSA forms part of a servo control system that positions the moveable transducer head over a particular track on the disk to read or write information from and to that track, respectively.
Typically, a conventional hard disk drive includes a disk having a plurality of concentric tracks. Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information may be provided on each disk to determine the position of the moveable transducer head.
The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo sectors or wedges that are angularly spaced from one another and are interspersed between data sectors around each track of each disk. Each servo wedge typically includes at least a phase lock loop (PLL) field, a servo synch mark (SSM) field, a track identification (TKID), a wedge ID field having a binary encoded wedge ID number to identify the wedge, and a group of servo bursts (e.g. an alternating pattern of magnetic transitions) which the servo control system of the disk drive samples to align the moveable transducer head with or relative to a particular track. Typically, the servo control system moves the transducer head toward a desired track during a course “seek” mode using the TKID field as a control input. Once the moveable transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the moveable transducer head over that track in a fine “track follow” mode. During track following mode, the moveable transducer head repeatedly reads the wedge ID field of each successive servo wedge to obtain the binary encoded wedge ID number that identifies each wedge of the track. In this way, the servo control system continuously knows where the moveable transducer head is relative to the disk.
Today, disks, especially for small form factor (SFF) disk drives, are increasingly being servo-written by external media servo writers before being assembled into disk drives. During external media servo-writing, multiple disks are simultaneously servo-written to without having to be located in a disk drive. The external media servo writer typically controls a rotatable actuator assembly including actuator arms having one or more heads respectively attached to each actuator arm, in which the actuator assembly rotates about a pivot such that the heads are radially positioned over the disks, respectively, in order to write servo sectors onto the disk based upon a timing clock.
When the externally servo-written disks are later assembled into a disk drive, servo wedge misalignment often occurs due to the different mechanical characteristics of the actuator assembly of the external servo writer and that of the disk drive. In particular, disk slip often occurs which is caused by gap differences in the mechanical hub of the disk drive versus that of the external servo writer. Further, disk slip may occur due to shock forces that have been applied to the disk drive.
In particular, the servo wedges of the disk may be misaligned relative to the rotating center of the disk because of disk slip. Thus, disk slip creates a radial displacement error for the head during track following and read/write operations. This radial displacement error of the head further varies dependent upon the track zone of the disk. For example, outer diameter (OD) and inner diameter (ID) track zones may have different radial displacement errors in response to skew angles. In the presence of large disk slip, the head may sway back and forth during track following mode, often over many tracks. These effects are intensified due to the micro-jogging displacement of the read and write elements of the head. Because of this, a great deal of position and timing uncertainties are introduced into the servo control system thereby causing problems during track following and read/write operations.
More particularly, due to this servo wedge misalignment, the servo control system may be very inefficient in track-following resulting in long time delays, and in the worst case, may not be able to consistently lock onto servo sectors during track following resulting in the failure of the disk drive. Further, in the presence of large disk slip, over track writing may occur resulting in disk drive errors.